Wireless monitoring and identification using spatially inhomogeneous structures

ABSTRACT

Tags encode information by means of spatial inhomogeneities that may be detected in the time domain; in effect, characteristics in space are transformed into time for sensing purposes. Such tags may be very inexpensively produced yet carry appreciable quantities of data. The inhomogeneities may be obtained by simple physical modifications to, or externally applied field biases operating on, materials that are very inexpensive to procure.

This is a continuation of application Ser. No. 09/617,249, filed on Jul.14, 2000, now U.S. Pat. No. 6,472,987 the entire disclosure of which isincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to remote sensing, tracking, andidentification, and in particular to the production and use ofinexpensive ID “tags.”

BACKGROUND OF THE INVENTION

Various monitoring technologies are known and used to monitor thelocation of an article or to provide identification in a wide range ofcontexts.

One such technology, known as “tagging,” is commonly employed, forexample, in shoplifting security systems, security-badge access systemsand automatic sorting of clothes by commercial laundry services.Conventional tagging systems may use some form of radio-frequencyidentification (RF-ID). In such systems, RF-ID tags and a tag reader (orbase station) are separated by a small distance to facilitate near-fieldelectromagnetic coupling therebetween. Far-field radio tag devices arealso known and used for tagging objects at larger distances (far-fieldmeaning that the sensing distance is long as compared to the wavelengthand size of the antenna involved).

The near-field coupling between the RF-ID tag and the tag reader is usedto supply power to the RF-ID tag (so that the RF-ID tag does not requirea local power source) and to communicate information to the tag readervia changes in the value of the tag's impedance; in particular, theimpedance directly determines the reflected power signal received by thereader. The RF-ID tag incorporates an active switch, packaged as a smallelectronic chip, for encoding the information in the RF-ID tag andcommunicating this information via an impedance switching pattern. As aresult, the RF-ID tag is not necessarily required to generate anytransmitted signal.

Even though RF-ID tags have only a small and simple electronic chip,they are relatively complex devices requiring sophisticatedmanufacturing techniques to produce. A simpler alternative involvesmarker elements adapted to affect an interrogation signal in ameasurable, characteristic way. Many such systems involve magnetic ormagnetomechanical tags. For example, a magnetic wire or strip exhibitingharmonic behavior may be stimulated within an interrogation zone bytransmitter antenna coils. The coils generate an alternating magneticinterrogation field, which drives the marker into and out of saturation,thereby disturbing the interrogation field and producing alternatingmagnetic fields at frequencies that represent harmonics of theinterrogation frequency. The harmonics are detected by receiver antennacoils, which may be housed in the same structure as the transmittercoils. Accordingly, the appearance of a tagged article within thezone—which may be defined, for example, near the doors of a retail storeor library—is readily detected.

While inexpensive, magnetic antitheft systems tend to encode verylittle, if any, information. Essentially, the tag merely makes itspresence known. While some efforts toward enhancing theinformation-bearing capacity of magnetic tags have been made—see, e.g.,U.S. Pat. Nos. 5,821,859; 4,484,184; and 5,729,201, which disclose tagscapable of encoding multiple bits of data—the tags themselves tend to becomplex and therefore expensive to produce, and may require specialdetection arrangements that limit the interrogation range (the '859patent, for example, requires scanning a pickup over the tag) or involvespecialized equipment.

DESCRIPTION OF THE INVENTION BRIEF SUMMARY OF THE INVENTION

The present invention utilizes tags having spatial inhomogeneities thatencode information, and which may be detected in the time domain; ineffect, characteristics in space are transformed into time for sensingpurposes. Such tags may be very inexpensively produced yet carryappreciable quantities of data. Unlike the prior art, which requiresspecialized information-bearing structures, the present invention canutilize simple physical modifications to, or externally applied fieldbiases operating on, materials that are very inexpensive to procure.

A first embodiment utilizes an elongated, amorphous, magneticallysusceptible element, such as a magnetic wire. Along the length of theelement, responsiveness to a time-varying magnetic field is altered in aspatial pattern corresponding to the information to be encoded. Theelement is then subjected to an interrogating magnetic field, and itsresponse sensed over time to recover the spatially encoded information.It should be stressed that the harmonic tags described earlier can alsotake the form of magnetic wires that are subjected to interrogationsignals. In such traditional systems, however, the signals are sensed inthe frequency domain, not the time domain in order to provide acharacteristic signature rather than information. The harmonics, inother words, merely facilitate unambiguous detection in anelectromagnetically noisy environment.

Alternatively, instead of sensing the response of the element over time,amplitude and phase are detected and the time-domain informationrecovered from the phase. This is once again in contrast to traditionalsystems, which neither preserve nor analyze phase information.

In a second embodiment, the element exhibits magnetoelastic behavior,and once again the element is selectively modified either physically orby application of bias fields in accordance with a pattern ofinformation. The element's response to an interrogating magnetic fieldis sensed over time to recover the encoded information. For example,discrete bias fields applied to the element may define, along the lengthof the element, a plurality of segments responding differently to theapplied field and producing intermodulating response signals. Theseresponse signals are sensed and analyzed in the time domain tocharacterize the bias fields and thereby read the information theyencode.

Once again, magnetoelastic markers have previously been used for taggingpurposes, but in a manner very different from that described herein. Inparticular, prior-art surveillance systems utilize only the fundamentalmechanical resonance frequency of the marker. A representative markerincludes one or more strips of a magnetoelastic material packaged with amagnetically harder ferromagnet (i.e., one with a higher coercivity)that provides a biasing field to establish peak magnetomechanicalcoupling. The mechanical resonance frequency of the marker is dictatedessentially by the length of the strip(s) and the biasing fieldstrength. When subjected to an interrogating signal tuned to thisresonant frequency, the marker responds with a large signal field thatis detected by a receiver. The size of the signal field is partiallyattributable to an enhanced magnetric permeability of the markermaterial at the resonance frequency.

In other prior-art systems, the marker is excited into oscillations bysignal pulses, or bursts, generated at the marker's resonance frequencyby a transmitter. When an exciting pulse ends, the marker undergoesdamped oscillations at its resonance frequency (i.e., the marker “ringsdown”), and this response (ring down) signal is detected by a receiver.Accordingly, prior systems generally involve a single resonancefrequency dictated by the entire tag structure, and a uniform biasfield.

In a third embodiment, the element is a higher-frequency element (e.g.,a UHF or microwave antenna) that is selectively modified eitherphysically or by application of bias fields in accordance with a patternof information. The modifications cause modulation to be introduced intothe received signal, and the pattern of modulations is indicative of themodifications (and therefore the encoded information).

In a fourth embodiment, magnetic inhomogeneities are established withrespect to an otherwise uniform NMR-responsive sample; for example, amagnetic bias strip may be disposed near or against the sample. Thepattern of magnetic biases results in an NMR spectrum with multiplepeaks corresponding to the bias pattern.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing discussion will be understood more readily from thefollowing detailed description of the invention, when taken inconjunction with the accompanying drawings, in which:

FIG. 1 shows a hysteresis loop illustrating the performance of amagnetic material in accordance with the present invention;

FIG. 2A is a plan view with an enlarged region showing operation of theinvention using a magnetic hysteretic element acted upon by externalmagnetic bias fields from a conventional magstripe;

FIG. 2B is an isometric view of an alternative source of magnetic biasfields;

FIG. 3 is a detector circuit suitable to operate the embodimentillustrated in FIG. 2;

FIG. 4 is an enlarged depiction of a magnetic hysteretic element thathas been physically modified to achieve spatial inhomogeneities;

FIG. 5A is an enlarged depiction of a magnetoelastic element acted uponby external magnetic bias fields;

FIGS. 5B and 5C illustrate, respectively, the ring down of isolated andinteracting magnetoelastic resonance elements;

FIG. 6A is an isometric view of a segment of a microstrip antenna usefulin accordance with the present invention;

FIG. 6B is a plan view of a segment of a microstrip antenna withresonators or harmonic strips adjacent to the microstrip element;

FIG. 6C is a plan view of a microstrip element having physicaldiscontinuities that cause variations in impedance;

FIG. 6D is a sectional view of a microstrip antenna in which magneticbias is used to vary impedance;

FIG. 6E shows the time-domain effect of selective, local impedancevariations on the output signal;

FIG. 6F shows the phase effect of selective, local impedance variationson the output signal;

FIG. 6G is a block diagram of an arrangement for downconverting theoperation of a microwave-resonant tag to a lower frequency for purposesof detection; and

FIG. 7 schematically illustrates a detector circuit for the NMRembodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

1. Magnetic Hysteretic Embodiment

The first embodiment of the invention utilizes elongated, amorphous,magnetically susceptible elements, preferably those that magnetize bylengthwise propagation of a domain wall. These terms are best understoodwith reference to FIG. 1, which illustrates the performance of amagnetic material in terms of an M-H graph showing the material'smagnetic response to a changing applied field. The “magneticsusceptibility” of a material refers to the size of an applied magneticfield H necessary to induce a given degree of magnetization M in thematerial. In a demagnetized material, increasing the applied field Hincreases the magnetic induction M of the material along themagnetization line 110. Thus, the induction increases quickly as thefield H rises above zero; the more susceptible the material, the morequickly the line 110 will begin to rise. When the external field isdecreased, however, the magnetic induction retreats relatively slowlyalong the demagnetization line 120. This hysteresis reflects thetendency of a ferromagnetic material to retain an induced magnetization.Thus, when the applied field strength is reduced to zero, the materialstill retains a “remanent magnetization” M_(r). A reverse magnetic fieldmust be applied to return M to zero in a ferromagnetic material; thestrength of that field, H_(c), is termed the “intrinsic coercivity” ofthe material. Substantially square hysteresis loops, such as that shownin FIG. 1, are preferred for the present invention for the reasonsdiscussed below. These occur due when the magnetizable material exhibitsmagnetic anisotropy—i.e., the tendency for magnetization to lie alongparticular axes. This results in relatively sharp transitions along thehysterisis loop, since magnetization tends to flip rather than assumeintermediate directions.

An amorphous material contains no widespread crystalline structure orlong-range order. As a consequence, local changes in magnetization tendto remain confined to the affected region without propagation; moreover,bulk structural properties do not overwhelm properties intrinsic to thematerial itself.

An elongated, amorphous, magnetically susceptible element useful in thepractice of the present invention may take the form of a wire (as setforth, for example, in U.S. Pat. No. 5,554,232, the entire disclosure ofwhich is hereby incorporated by reference). Such a wire will have anatural magnetic orientation along its entire length. If the wire isthin—i.e., if the cross-sectional diameter is small than the width of amagnetic domain—exposure of the material to a magnetic field ofsufficient strength results in magnetization propagating as a domainwall along the length of the wire from one end to the other, flipping orreinforcing the wire's natural magnetic orientation. (A magneticinterrogation field ordinarily has some degree of non-uniformity, sothat magnetization propagates along the wire in a single direction asdictated by the field divergence.)

In accordance with the invention, the element may be biased so that thenet magnetizing force varies over the length of the material in apattern representative of information; or the element may instead bephysically modified so that its magnetic susceptibility varies over itslength. FIGS. 2A and 2B illustrate the first of these approaches. Withreference to FIG. 2A, magnetic stripe (“magstripe”) 230, permanentlyencoded with magnetic information, is placed in sufficient proximity toa magnetic wire 235 for the fields from magstripe 230 to exert ameasurable effect thereon. Magstripe 230 contains a series of zones,representatively indicated at 240, biased in one or the other directionrelative to wire 235. The bias fields 245 created by zones 240 eitherretard or enhance magnetic induction in wire 235 within the spatialregions over which the zones have an effect. In other words, sincemagnetization occurs in accordance with the wire's intrinsic hysteresisloop, bias fields 245 will either complement the applied field (somagnetization within the region affected by the field occurs morequickly than in unaffected regions) or oppose it (so magnetizationoccurs less quickly). As a result, the rate of propagation of the domainwall through wire 235 will vary, with the pattern of variation dictatedby the pattern of bias fields 245. Following the progress ofmagnetization over time, therefore, indicates the distribution of biasfields 245 over the length of wire 235.

An alternative to a magstripe bias strip is shown in FIG. 2B. In thiscase, the structure 250 comprises a strip 255 of a uniformly magnetizedmaterial in contact with a support 260 of generally equivalentdimensions. The bulk of support 260 has a first magnetic permeability,while a series of selectively placed regions 265 have a secondpermeability. As a result, the magnetic field experienced by a wirelocated below support 260 will vary with position. For example, support260 may be a highly permeable material with regions 265 representing airgaps, resulting in lower permeability. More typically, regions 265 willhave a higher permeability than the bulk support 260 so that thestructure 250 behaves in the manner of a magstripe. For example, support260 may be plastic and regions 265 embedded strips of iron ormagnetically permeable ink compatible with a printed manufacturingprocess.

A suitable detector circuit for this purpose is shown in FIG. 3. Thecircuit includes an energizing or excitation coil 305; an oscillator310, which provides AC signals that create the excitation field; adetection coil 315; and a computer 320 that controls oscillator 310 andinterprets the signals sensed by detection coil 315. As long as themagnetic field produced by coil 305 is sufficiently strong, theorientation of wire 100 with respect to the field will be unimportant;the effective field strength is represented by the dot product of thefield vector H with the direction of the wire. In general, effectivefield strengths as small as 1 gauss are sufficient to magnetize a thinwire 235; excitation frequencies are generally below 1 MHz, andtypically range from as few as 5 Hz to 10 kHz. A square hysteresis loopresults in discrete magnetization “jumps” that are discrete andtherefore more easily detected as a series of bits.

It should also be noted that magstripe 230 may be the conventional typeof strip applied to credit cards or the like, or may be a strip ofstronger magnetic material (or a series of discrete magnets) to providegreater field strength, or some combination thereof. As used herein, theterm “magstripe” is intended to connote any of these approaches, and theterm “magnetic bias strip” is intended to embrace magstripes and otherstructures exerting similar effects, such as the structure 250 shown inFIG. 2B.

In the second approach to practice of this embodiment, discrete physicalmodifications are introduced along the length of wire 235. Thesemodifications selectively retard or enhance magnetic induction in theregions over which they extend. FIG. 4 shows a pair of representativemodifications 410, 415 in wire 235. These may be, for example, holesthrough or indentations in wire 235 (produced, for example, by chemicaletching or laser cutting), which act as domain wall barriers or producedamping; or nucleation sites or magnetization points (applied, forexample, by spot welding) that enhance magnetic susceptibility orpermeability.

2. Magnetoelastic Embodiment

In the second embodiment of the invention, an illustrative fragment ofwhich appears in FIG. 5A, an elongated, amorphous, magnetoelasticelement 510 is positioned proximate to a magnetic bias strip 520. Onceagain, element 510 may be a magnetic wire as described above. Magneticbias strip 520 contains a series of zones, representatively indicated at525, 530, of different widths. Each zone 525, 530 creates a distinctbias field 535, 540, and because element 510 is amorphous, the effectsof bias fields 535, 540 are largely confined to the opposed regions 545,550 of element 510. Each region 545, 550 behaves essentially as aseparate resonance element with its own resonant frequency. During thering-down phase following an excitation signal, e.g., from excitationcoil 305 of the detector circuit shown in FIG. 3, the signals from thevarious biased regions interact and modulate one another. Observing themodulations in the time domain facilitates extraction of the pattern ofbiased regions. This is illustrated in FIGS. 5B and 5C. FIG. 5B showsthe ring-down phase of a single (unbiased) resonance element, in whichthe oscillations 560 decay within a smooth envelope 565. In FIG. 5C, theinteractions or “beating” among the oscillations 560 from differentresonance elements (corresponding to biased and unbiased regions of theelement 510) produce a patterned envelope 570, which encodes the patternof the bias zones.

Alternatively, the differently responsive regions 545, 550 can be formedby physical modification rather than by field biasing. For example,mechanically separate regions can be defined along element 510 bycrimping, cutting, effectively resulting in individual resonators whoseresponse signals interact as described above.

3. Microstripline Embodiment

With reference to FIG. 6, a microstrip line antenna 600 includes agroundplane electrode 610, a dielectric spacer 615, and a microstripelement 620. In ordinary usage, antenna 600 acts as a waveguide,receiving broadcast signals in the microwave or UHF range of theelectromagnetic spectrum for amplification and use. In accordance withthe present invention, discontinuities are introduced along the lengthof microstrip element 620. When antenna 600 is excited in its resonantmode by an appropriate electromagnetic signal, the discontinuities arecoupled into the antenna's response, affecting its impedance and causingenergy to leak out of the structure at the points of discontinuity.

As a result, the detected signal will contain modulations not present inthe excitation signal, and these modulations are used to reconstruct thepattern of discontinuities. The discontinuities may be caused bymagnetic bias fields, by additional waveguide elements, or by a seriesof physical modifications 630. In one approach, illustrated in FIG. 6B,a series of microwave resonators or harmonic strips 632 are mountedadjacent microstrip element 620; these alter the effective impedance ofelement 620 along the zones of adjacency.

In FIG. 6C, physical modifications—such as a series of notches 634—areused to alter the impedance of element 620. Since the impedance ofelement 620 is determined, inter alia, by its width, notches 634increase impedance locally where they occur.

The impedance of microstrip element 620 can also be altered locally byselectively applied magnetic fields. As shown in FIG. 6D, a magneticbias element 635 (similar in concept to the structure 250 shown in FIG.2B) can underlie element 620. Element 635 includes a uniformlymagnetized layer 637 disposed on dielectric spacer 615, and a series ofmagnetically permeable segments 636 upon which microstrip element 620rests. The magnetic field experienced by element 620 will be moreintense where the element is in contact with segments 636, since theseelements are more permeable than air (which otherwise intervenes betweenmagnetized layer 637 and element 620). The magnetic field variesimpedance along microstrip element 620 in the manner of the physicaldiscontinuities shown in FIG. 6C.

The effect of any of these modifications on the output signal is shownin FIGS. 6E and 6F. FIG. 6E, which plots output amplitude as a functionof time, shows a series of peaks that result from the impedancemismatches along element 620. FIG. 6F illustrates the effect on thephase of the output signal; the signal amplitude will vary with phase asdetermined by the pattern of impedance mismatches. Thus, the pattern ofthose discontinuities generates a unique effect on the output signalwhich can be observed in the time domain or as a function of phase,facilitating recovery of the pattern.

In the frequency domain, the consequences of modulation typically extendthrough numerous harmonics into very high frequency levels. This resultsin a large affected bandwidth, which may pose difficulties in terms ofsignal radiation from the antenna. To avoid this effect, the approachshown in FIG. 6G may be adopted, whereby antenna 600 is coupled into asecondary response element 640 by means of a mixer 645. Secondaryelement 640 has a lower-frequency response characteristic, and as aresult, the modulations of interest are downconverted to this lowerfrequency band for sensing by a detector 650.

For example, secondary element 640 may be the amorphous, magneticallysusceptible element described above in connection with the hystereticembodiment. In this case, mixer 645 may be a nonlinear mixing component,such as a diode, which receives the output of antenna 600. The output ofmixer 645 is then converted to a magnetic signal (via a coil), which iscoupled into the magnetically susceptible element 640. Element 640 isdriven as described previously (e.g., using, as the detector 650, thecircuit illustrated in FIG. 3). As a result of this arrangement, mixer645 multiplies the output of antenna 600 with the driving signal from alocal excitation coil 660. Consequently, the signals produced by thelocal magnetic phenomena in element 640 will produce detectablemodulations in the higher-frequency signal coupled by antenna 600.Alternatively, if the power of the high-frequency signal is sufficient,then adequate power will be available after the mixer stage in order toexcite the magnetic element without the need for the additional localexcitation coil.

4. NMR Embodiment

In a time-invariant applied magnetic field B_(DC)=B₀z along the z axis,a nucleus of spin I has 2I+1 possible quantum spin states at equallyspaced corresponding energy levels, any pair of levels being separatedby ${\Delta \quad E} = \frac{\mu \quad B_{{^\circ}}}{I}$

corresponding to the resonant frequency$W = {\frac{\Delta \quad E}{h}.}$

The magnetic moment of the nucleus$\mu = \frac{\gamma \quad {hI}}{2\pi}$

includes the magnetogyric moment γ, which is a constant for a givennucleus. A nucleus of spin $I = \frac{1}{2}$

may thus be in either of two spin states, i.e., having its magneticmoment component aligned with or against the applied magnetic field.

An rf electromagnetic field of frequency ω and phase Φ applied in adirection perpendicular to the static field (e.g., along the xdirection) introduces a magnetic field of magnitude B₁ that can beviewed as a single field rotating about the z axis at ω. In a frame ofreference rotating with this magnetic field, the spin experiences aneffective magnetic field having the components (B₁ cos Φ)x, (B₁sin Φ)y,and $\left( {B_{{^\circ}} - \frac{\omega}{\gamma}} \right){z.}$

Thus, the spins absorb energy most strongly at the resonant frequency W.Because that frequency is a function of ΔE, which itself depends on theapplied static field B₀, the application of magnetic biases along thelength of an NMR-responsive sample will alter the local resonantfrequency of the sample by augmenting or reducing the static field B₀.

This may be exploited as illustrated in FIG. 7. A pair of magnets 710,715 produce a static magnetic field B_(DC). A uniform, NMR-responsivesample 720 is positioned within the field B_(DC), and a magnetic biasstrip 725 is disposed near or against sample 720; the sample 720 andbias strip 725 together form a tag in accordance with the presentinvention. A coil 730 encircles or, as shown, may be located beneathsample 720 and bias strip 725. A source 735 of rf electromagneticexcitation energizes coil 730, resulting in a time-varying magneticfield B_(RF) whose direction is perpendicular to that of B_(DC). Areader 740 detects the energy absorbed by sample 720. Without bias strip725, a single dominant peak (corresponding to the resonant frequency Wof sample 720) would be observed as source 735 sweeps through a range offrequencies. However, because of the pattern of alternating magneticbiases imposed by strip 725, the NMR spectrum reveals multiple peakswhose pattern corresponds to the bias pattern. Although this pattern isobservable in the frequency domain, better signal-to-noise ratios areobtained if the excitation is carried out through a sequence of rfpulses and then the frequency response of the tag is detected as afunction of time, using so-called “spin-echo” techniques.

It will therefore be seen that the foregoing represents an inexpensiveand versatile approach to encoding information for external sensing. Theterms and expressions employed herein are used as terms of descriptionand not of limitation, and there is no intention, in the use of suchterms and expressions, of excluding any equivalents of the featuresshown and described or portions thereof, but it is recognized thatvarious modifications are possible within the scope of the inventionclaimed.

What is claimed is:
 1. A method of sensing information, the methodcomprising the steps of: a. providing a magnetically responsive,amorphous, magnetoelastic element having a length; b. varying, along thelength of the element, the responsiveness of the element to an appliedtime-varying magnetic field in a spatial pattern corresponding to theinformation, such that a propagation rate of a magnetic response alongthe length will vary in accordance with the spatial pattern; c.subjecting the element to an applied time-varying magnetic field toinduce the magnetic response therein; and d. sensing changes in theresponse propagation rate to recover the spatially encoded informationby removing the field and receiving a response signal from the elementto recover the spatially encoded information.
 2. The method of claim 1wherein the element has a length and responsiveness is varied byapplying, along the length of the element, discrete bias fields to theelement.
 3. The method of claim 2 wherein the bias fields define, alongthe length of the element, a plurality of segments respondingdifferently to the applied field and producing intermodulating responsesignals, the sensing step comprising identifying the response signals tocharacterize the bias fields.
 4. The method of claim 1 wherein theresponsiveness is varied by introducing discrete physical modificationsalong the length of the element.
 5. The method of claim 4 wherein themodifications define, along the length of the element, a plurality ofsegments responding differently to the applied field and producingintermodulating response signals, the sensing step comprisingidentifying the response signals to characterize the modifications.
 6. Asystem for encoding and sensing information, the system comprising: a. afield-responsive, elongated, amorphous, magnetoelastic element having alength along which is varied the responsiveness of the element to anapplied time-varying magnetic field, the variation conforming to aspatial pattern corresponding to the information, such that apropagation rate of a magnetic response along the length will vary inaccordance with the spatial pattern; and b. means for sensing atime-dependent response of the element to an applied time-varyingmagnetic field to recover the spatially encoded information by removingthe field and receiving a response signal from the element.
 7. Thesystem of claim 6 further comprising means for applying a series of biasfields applied along the length of the element.
 8. The system of claim 7wherein the bias fields define, along the length of the element, aplurality of segments responding differently to the applied field andproducing intermodulating response signals, the sensing meansidentifying the response signals to characterize the bias fields.
 9. Thesystem of claim 6 wherein the element comprises discrete physicalmodifications along the length thereof.
 10. The system of claim 9wherein the modifications define, along the length of the element, aplurality of segments responding differently to the applied field andproducing intermodulating response signals, the sensing meansidentifying the response signals to characterize the modifications.